The invention relates to an electro-optical device for optically scanning an information carrier, comprising a stationary frame, a movable optical holder in which an objective having an optical axis is eccentrically mounted, which holder is translatable along and pivotable about a pivotal axis which extends parallel to and is spaced from the optical axis, which holder is further provided with a balancing mass, bearing or supporting means for movably supporting the optical holder, and a first electromagnetic actuator means for translating the optical holder along the pivotal axis and a second electromagnetic actuator means for pivoting the optical holder about the pivotal axis, each of the actuator means comprising a first actuator element secured to the frame and a second actuator element secured to the holder, the first actuator element and the second actuator element being adapted to cooperate with each other via air gaps.
Electro-optical devices of this type are known in many versions, for example from European Patent Application EP-A 0,121,857 (herewith incorporated by reference), to which U.S. Pat. No. 4,566,089 corresponds. The electro-optical device known from said Patent Application is intended for use in an optical-disc player. The electro-optical device enables recording tracks to be recorded in and/or to be read from a recording surface of an optical disc by means of a radiation beam which is produced by a laser source and which is concentrated to form a radiation spot. The holder of the electro-optical device, which is of rectangular construction, is movably suspended relative to the frame with the aid of flexible bearing means in the form of four springs having two bends each. A coil is arranged around the holder to enable the holder to be translated along the optical axis of the objective to maintain the radiation spot constantly in focus on the recording surface of the optical disc. The translational movements of the objective will be referred to hereinafter as focussing movements.
In addition to said focussing movements of the objective radial and tangential movements are necessary for exactly positioning the light spot relative to the tracks on the disc, which movements are generally referred to as radial and tangential tracking movements. In order to produce the radial tracking movements a further coil is arranged on the afore-mentioned coil at each of the four corners of the holder. Said coils all cooperate magnetically with magnets arranged on the stationary frame. When the objective performs a radial tracking movement the holder is rotated about an additional bearing means comprising a spindle which is secured to the frame and a bearing bush which is mounted in the holder so as to be movable relative to said spindle.
A disadvantage of the known electro-optical device is that its construction necessitates the use of an additional element serving as a balancing mass in order to guarantee a reliable operation of the device. Obviously such an additional element results in an increase in weight of the holder and consequently an increasee in the power needed for the focussing and radial tracking movements. Moreover, this necessitates the use of coils which are heavier than is strictly necessary, which leads to a further increase in weight of the entire holder and which may increase in weight of the entire holder and which may also result in an unnecessarily large amount of heat being generated.
European Patent Application EP-A 0,063,934 (herewith incorporated by reference), to which with U.S. Pat. No. 4,467,463 corresponds, describes another construction of the electro-optical device defined in the opening paragraph. This known device employs an objective comprising a plurality of lenses, which objective is mounted in a holder which is pivotable about a spindle journalled in a frame and which is translatable along this spindle. The holder is provided with a magnet which extends in an opening of a coil former provided with coils in such a way that magnetic forces which act parallel to or transversely of said spindle are exerted on the magnet when the coils are driven selectively. Said forces serve to provide the focussing movements and the tracking movements of the objective. A disadvantage of this known device is that the point of action of the magnetic forces intended for the focussing movements is spaced from the axis of rotation, which is liable to give rise to tilting effects between the holder and the spindle during focussing of the objective, which may impair a correct operation of the device.